Process for crosslinking thermoplastic polymers with silanes employing peroxide blends and the resulting crosslinked thermoplastic polymers

ABSTRACT

A composition comprising at least one silane possessing an unsaturated organic function; at least two free radical initiators, and a process for producing silane-crosslinked thermoplastic polymers comprising providing a cross-linkable compound; at least one thermoplastic polymer; and, reacting the crosslinkable compound under reactive mechanical-working conditions and exposing the crosslinkable compound to moisture to provide crosslinked thermoplastic polymers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a composition comprising silane crosslinkersand two or more free radical initiators with different half-lifetemperatures and a process of crosslinking thermoplastic polymers withsilane crosslinker in the presence of a blend or mixture of free radicalinitiators. The invention further relates to the moisture crosslinkedthermoplastic polymers resulting from the process.

2. Description of Related Art

For many applications, e.g., wire and cable insulation,weatherstripping, fibers, seals, gaskets, foams, footware, flexibletubing, pipes, bellows, tapes, etc., certain selected properties (e.g.tensile strength, compression set, thermal and chemical resistance) ofarticles manufactured from one or more thermoplastic polymers can beenhanced by introducing chemical linkages between the polymericmolecular chains which constitute the polymer, during or preferablyfollowing, the shaping or molding process. These chemical linkagesbetween different polymeric molecular chains are commonly referred to as“crosslinks”. Crosslinks can be introduced between different molecularchains of a thermoplastic polymer by a number of mechanisms, one ofwhich is to graft to the individual polymer backbones or chains thatconstitute the bulk polymer with a chemically reactive compound in sucha manner that the grafted compound on one backbone may subsequentlyreact with a similar grafted compound on another backbone to form thecrosslink. Exemplary of this process is the “silane crosslinking”process.

This process employs a silane-containing compound that crosslinks thesethermoplastic polymer compounds. Silanes can be grafted to a suitablethermoplastic polymer by the use of a suitable quantity of organicperoxide or other free radical initiator, either before or during ashaping or molding operation. Additional ingredients such asstabilizers, pigments, fillers, catalysts, processing aids etc., mayalso be included in the mixture.

When using silane-peroxide blends for thermoplastic polymercrosslinking, a compromise must be made between grafting efficiency andprocess efficiency, such as extrusion rate and run times. The formationof a cross-linkable material by this means is, however, difficult tocarry out since it requires critical control of the process. If theprocess is carried too far, the thermoplastic polymer may partiallycross-link and solidify in the processing apparatus, for example anextruder, with consequent difficulties in achieving consistent goodquality products and delays involved in removing the partiallycross-linked product from the processing equipment. Care must also beexercised to ensure that articles prepared from the polymer retain theirshape during subsequent heating to bring about the cross-linkingprocess.

It has been observed that gel formation, screw-build up and scorchingmay result when using highly reactive silane-peroxide blends. This isparticularly significant for processes using conditions and processingequipment that impose severe melting and mixing conditions leading tohigh shearing stresses in the polyolefin. These problems generally arisedue to early and eventually complete activation of the peroxide duringthe initial melting and homogeneisation process. The prior art has dealtwith these problems by using less reactive silane blends but thisapproach can diminish the grafting efficiency of the crosslinkablethermoplastic polymers.

U.S. Pat. No. 3,646,155 describes the crosslinking of polyolefins by thereaction of polyolefin with an unsaturated hydrolysable silane at atemperature above 140° C. in the presence of a compound capable ofgenerating free radical sites in the polyolefin.

U.S. Pat. No. 4,252,906 describes a process using a crosslinkablepolyethylene resin composition comprising a silane-modified polyethyleneresin and an organic peroxide.

U.S. Pat. No. 4,117,195 describes what is commercially known as themonosil process of making a crosslinked extruded product frompolyethylene or other suitable polymer.

U.S. Pat. No. 4,412,042 describes a process for preparing polyolefinscross-linked by a silane linkage. This is accomplished by reacting asilane with an ethylene/α-olefin copolymer of a specific density in thepresence of a free radical generating agent, e.g., a benzoyl peroxide.

U.S. Pat. No. 5,112,919 describes what is commercially known as theXL-Pearl process involving the solid feeding of silane cross-linkingagents into an extruder.

U.S. Pat. No. 5,741,858 discloses a silane-crosslinked blend comprisinga polyolefin elastomer, a crystalline polyolefin polymer, a silanecrosslinker and preferably a peroxide initiator.

U.S. Pat. No. 5,744,553 describes what is commercially known as thespherisil process.

U.S. Pat. No. 6,455,637 discloses a coating material formed by theprocess of reacting a polyolefin with a silane in the presence of afree-radical initiator.

EP 1288235 describes a composition for cross-linked polymers thatexhibit scorch and gel formation reduction properties. This reference isincorporated by reference herein.

There remains a need for a means of crosslinking polyolefins and othersilane crosslinkable thermoplastic polymers under reactivemechanical-working conditions using silane crosslinkers and free radicalinitiators while minimizing such aforenoted problems as gel formation,screw-buildup and/or scorching while maintaining a high level ofgrafting (crosslinking) efficiency.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a compositioncomprising silane crosslinkers and two or more free radical initiatorswith different half-life temperatures and a process of crosslinkingthermoplastic polymers by reacting thermoplastic polymers with silanesand/or a blend of silanes and a blend of free radical initiators underreactive mechanical-working conditions.

It is a further object of the invention to provide a process ofcrosslinking thermoplastic polymers under reactive mechanical-workingconditions that will minimize the occurrence of gel formation,screw-buildup and/or scorching while maintaining high crosslinkingefficiency.

In keeping with these and other objects of the invention, there isprovided a composition comprising:

-   -   (i) at least one silane possessing an unsaturated organic        function;    -   (ii) at least two free radical initiators, the first initiator        having a first half-life temperature and the second initiator        having a second half-life temperature, said second half-life        temperature being higher than said first half-life temperature;    -   (iii) optionally one or more condensation catalysts;    -   (iv) optionally, one or more stabilizers, stabilizer packages,        inhibitors or free radical scavengers; and,    -   (v) optionally, other additives such as fillers, colorants,        processing aids, etc.

In further keeping with these and other objects of the invention, thereis provided a process for producing silane-crosslinked thermoplasticpolymers comprising:

-   -   a) providing a cross-linkable compound comprising a mixture of:        -   (i) at least one silane possessing an unsaturated organic            function;        -   (ii) at least two free radical initiators, the first            initiator having a first half-life temperature and the            second initiator having a second half-life temperature, said            second half-life temperature being higher than said first            half-life temperature;        -   (iii) at least one thermoplastic polymer; and,        -   (iv) optionally, one or more condensation catalysts; and,        -   (v) optionally, one or more stabilizers, stabilizer            packages, inhibitors or free radical scavengers; and,        -   (vi) optionally, other additives such as fillers, colorants,            processing aids, etc; to produce a cross-linkable compound;            and,    -   b) reacting the mixture of step (a) under reactive        mechanical-working conditions and exposing the crosslinkable        compound to moisture to provide crosslinked thermoplastic        polymers.

The expression “reactive mechanical-working conditions” herein shall beunderstood to mean the conditions of elevated temperature and residencetime prevailing within a mechanical-working apparatus such as anextruder and exposure to moisture, such conditions being sufficient tobring about the reactive processing, allowing free radical initiatoractivation and silane grafting onto the thermoplastic polymer, of apolyolefin contained in a mixture of at least a thermoplastic polymer,silane crosslinker and free radical initiator(s).

The process of the present invention enables the crosslinking ofthermoplastic polymer to be carried out under less critical processingconditions than those which are normally obtained in connection withconventional peroxide crosslinking techniques. The present process alsoenables the reaction between the thermoplastic polymer, e.g.,polyolefins and the silane to be effected relatively quickly, if desiredin the absence of a solvent, and employing only minor amounts of silaneand free radical initiators.

The process of the invention therefore lends itself to the preparationof a cross-linked thermoplastic polymer in conventional extrusionequipment and under conditions and in a time comparable to thosenormally employed for the compounding of such materials. Furthermore,the improved grafting efficiency of the present invention decreases therequirement for expensive silane and free radical initiator reactants.

The present invention employs a blend of free radical initiatorspossessing a range of half-life temperatures such that undesired earlyand/or concentrated activation of the free radical initiators isdecreased and the broad range of half-life temperature allows for ahomogenous level of activation and grafting throughout the reactiveprocess which leads to an improved level of grafting efficiency andsilane crosslinking efficiency in the compound. In addition, the use offree radical initiators containing a range of half-life temperatures hasbeen observed to reduce the level of gel formation, screw buildup and/orscorching allowing for extended run-times.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The crosslinkable thermoplastic polymers of the present inventioninclude polyolefins, such as, one or more α-olefins, α-olefincopolymers, α-olefin terpolymers and mixtures thereof. Examples ofuseful polyolefins include high-pressure low-density polyethylene,medium/low-pressure high-density polyethylene, low-pressure low-densitypolyethylene, medium-density polyethylene, an ethylene-α-olefincopolymer, polypropylene, an ethylene-ethyl acrylate copolymer, anethylene-vinyl acetate copolymer, an ethylene-propylene copolymer, anethylene-propylene-diene terpolymer, an ethylene-butene copolymer,polymethylpentane-1, polybutene, chlorinated polyethylene, anethylene-vinyl acetate-chlorine terpolymer, and the like, and mixturesthereof.

The silane(s) with an unsaturated organic function can be anyconventionally available silane(s) used in the silane cross-linking ofpolymers as is well known in the art. Advantageously, the silane(s) canbe those of the general formula RR′SiY₂ wherein R represents amonovalently olefinically unsaturated hydrocarbon or hydrocarbonoxyradical, each Y represents a hydrolysable organic radical and R′represents an R radical or a Y radical, which is reactive with the freeradical sites generated in the polyolefin. Examples of such radicals arevinyl, allyl, butenyl, cyclohexenyl, cyclopentadienyl, cyclohexadienyl,

the vinyl radical being preferred. The group Y can represent anyhydrolysable organic radical for example an alkyoxy radical such as themethoxy, ethoxy and butoxy radicals, an acyloxy radical, for example theformyloxy, acetoxy or propanoyloxy radicals, oximato radicals,e.g.—ON═C(CH₃)₂, —ON═CCH₂C₂H₅ and —ON═C(C₆H₅)₂ or substituted aminoradicals, e.g. alkylamino and arylamino radicals, examples of which are—NHCH₃, —NHC₂H₅ and —NH(C₆H₅)₂. The group R′ may represent an R group ora Y group. Preferably, the silane will contain three hydrolysableorganic radicals, the most preferred silanes being vinyltriethoxysilaneand vinyl trimethoxysilane.

The process of the invention employs a blend of at least two freeradical initiators, preferably three, the first of which possesses afirst half-life temperature and the second of which possesses a secondhalf-life temperature greater than that of the first. The second 0.1hour half-life temperature of the second free radical initiator isbetween 5° and 110° C. greater than the 0.1 hour half-life temperatureof the first free radical initiator. Preferably the second 0.1 hourhalf-life temperature of the second free radical initiator is between30° to 90° C. greater than the 0.1 hour half-life temperature of thefirst free radical initiator. Most preferably the second 0.1 hourhalf-life temperature of the second free radical initiator is between45° and 70° C. greater than the 0.1 hour half-life temperature of thefirst free radical initiator.

The first free radical initiator is preferably a peroxide and possessesa relatively low 0.1 hour half-life temperature, e.g. of from about 80°C. to about 160° C. and preferably from about 90° C. to about 155° C. asmeasured in a dilute solution of the initiators in monochlorobenzene.Suitable first free radical initiators and their range of 0.1 hourhalf-life temperatures are set forth in Table I as follows.

TABLE I First Free Radical Initiator Range of 0.1 hour half-life FirstFree Radical Initiator Temperatures [° C.] Di(2,4-dichloro benzoyl) 93peroxide Dilauroyl peroxide 99 Dibenzoyl peroxide 113 1,1Di(tertbutylperoxy)- 128 3,3,5-trimethylcyclohexane Tert-Butylperoxybenzoate 142 Dicumyl peroxide 154

In addition, other first free radical initiators can include tert-butylperoxypivalate, tert-butyl peroxy-2-ethylhexanoate,di(tert-butylperoxy)cyclohexane, tert-butylperoxy-3,5,5-trimethylhexanoate, tert-butyl peroxyacetate, di-tert-amylperoxide, di(tert-butylperoxyisopropyl)benzene and2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, as well as any free radicalinitiator that is conventionally used or known.

The second free radical initiator, also preferably a peroxide, possessesa higher 0.1 hour half-life temperature than that of the first freeradical initiator, e.g., on the order of from about 125° to about 190°C. and preferably from about 140° to about 170° C. Suitable second freeradical initiators and their range of 0.1 hour half-life temperaturesare set forth in Table II as follows.

TABLE II Second Free Radical Initiators Range of 0.1 hour half-lifeSecond Free Radical Initiator Temperatures Tert-Butyl peroxybenzoate 142Dicumyl peroxide 154 Tert-butyl cumyl peroxide 1592,5-Dimethyl-2,5-di(tert- 164 butylperoxy)hexyne-3

Additional second free radical initiators can include tert-butylperoxyacetate, di-tert-amyl peroxide,di(tert-butylperoxyisopropyl)benzene,2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and di-tert-butyl peroxide,as well as any free radical initiator that is conventionally used orknown.

The crosslinking process can further include the use of a condensationcatalyst. A wide variety of materials which function as condensationcatalysts for silanes are known in the art and any of such materials maybe employed in the process of this invention. Such materials include forexample metal carboxylates such as dibutyltin dilaurate, stannousacetate, stannous octoate, lead naphthenate, zinc octoate,iron-2-ethylhexoate and cobalt naphthenate, organic metal compounds suchas the titanium esters and chelates, for example tetrabutyl titanate,tetranonyl titanate and bis(acetylacetonyl) di-isopropyl titanate,organic bases such as ethylamine, hexylamine, dibutylamine andpiperidine and acids such as the mineral acids and fatty acids. Thepreferred catalysts are the organic tin compounds, for example,dibutyltin dilaurate, dibutyltin diacetate, and dibutyltin dioctoate.

Stabilizers and radical scavengers are also advantageously employed inthe reaction of the blend of free radical initiators and silane(s).

In accordance with the process of this invention, the reaction betweenthe thermoplastic polymer and the silane is carried out employing anysuitable mechanical-working apparatus heretofore employed in theprocessing of polyolefins, e.g., a screw-type extruder, an internalBanbury mixer or a roll mill, provided, of course, that it results inbringing the composition to grafting temperature. The preferredapparatus for use in providing the crosslinkable polyolefin of thisinvention is an extruder adapted to effect a kneading or compoundingaction on its contents. Such extruder apparatus may include suchoptional features as a heating jacket to augment the heat producedwithin the extruder barrel and a vacuum port whereby any unreactedsilane can be removed.

The thermoplastic polymer, silane crosslinker, free radical initiatorsand other components can be brought together by any convenient means.For example, the silane can be introduced into the apparatus in whichthe reaction is to take place dispersed on the surface of thethermoplastic polymer or it can be metered directly into the apparatus.The free radical initiators can also be introduced by way of the surfaceof the thermoplastic polymer and, when possible, dissolved in thesilane. The silane and/or peroxide components can also be introduced asdry-silanes, absorbed on suitable mineral or organic carriers.

Reaction between the silane and thermoplastic polymer can be carried outat any suitable temperature between about the melting and about thedegradation temperature of the polyolefin. The actual reactiontemperature employed will normally be determined by considerations ofthe type of apparatus in which the reaction is performed and whereappropriate on the power input for the apparatus and the compoundviscosity profile. When the thermoplastic polymer is polyethylene, it ispreferred to perform the reaction at temperatures similar to thoseusually met with during the processing of polyethylene, that is fromabout 140° to about 260° C. for periods from about 0.5 to about 10minutes.

Crosslinking of thermoplastic polymer according to the process of thisinvention is accomplished in the presence of moisture. The moisturepresent in the atmosphere is usually sufficient to permit thecross-linking to occur but the rate of crosslinking may be hastened ifdesired by the use of an artificially moistened atmosphere or liquidwater.

The invention is applicable to all processes used for the manufacturingof silane crosslinkable compounds or products where the silane isgrafted onto the polymer backbone using radical grafting. Such processesinclude the One-Step Monosil process, the One-Step XL-PEarl process, theOne-Step Spherisil P process, the Two-Step Sioplas process, and theOne-Step Soaking process.

While any conventional method can be used to graft the silanecrosslinker to the thermoplastic polymer, one preferred method isblending the thermoplastic polymer(s) with the initiator in the firststage of a reactor extruder, such as a single screw extruder, preferablyone with a length/diameter (L/D) ratio of about 25:1 or greater. Thegrafting conditions can vary greatly depending on the compoundformulation, but the melt temperatures are typically between about 160°and about 240° C., preferably between about 210° and about 230° C.,depending upon the residence time and the half-life of the initiator.

The articles prepared from the crosslinked compositions of thisinvention can be filled or unfilled. If filled, then the amount offiller present should not exceed an amount that would cause degradationof the properties of interest in crosslinked composition. Typically, theamount of filler present is between about 0 and about 80 weight percent,preferably between about 20 and about 60 weight percent based on theweight of the composition. Representative fillers include kaolin clay,magnesium hydroxide, aluminum trihydroxide, silica and calciumcarbonate. In a preferred embodiment of this invention in which a filleris present, the filler is coated with a material that will prevent orretard any tendency that the filler might otherwise have to interferewith the silane cure reaction. Stearic acid or silane coupling agentsare illustrative of such a filler coating.

Other additives can be used in the preparation of and be present in thearticles prepared from the crosslinked compositions of this invention,and includes antioxidants, processing aids, oils, plasticizers, pigmentsand lubricants.

The amounts of the various components of the present invention can varygreatly depending on the nature of the polyolefin and other componentsand the process of production of articles made from the silanecrosslinked polyolefin(s). Preferably, the silane(s) and peroxides willbe premixed, eventually with the catalyst(s), stabilizer(s), processingaid(s) and metal deactivator(s), and will be used at loading levels offrom about 0.2 to 3 weight percent of the total crosslinkable compound.The composition of the silane(s), peroxides and eventual other additivesshall preferably be characterized by the silane with an organic functionpresent at loading levels of from about 50 to about 99.9% and morepreferably from about 75 to about 98%. The blend of radical initiatorswill vary in amount as described above, depending on the desired rangeof radical initiator half-life temperatures and times. Preferably, inthe present invention, the amount of radical initiators that arepre-blended in the silane will be in total weight of the blend fromabout 0.05 to about 15% and more preferably from about 2 to about 8%.The level of catalysts can be present in an amount of from 0 to about10% and preferably from about 1 to about 5%. The total level of otheradditives which can preferably be included is from about 0 to about 25%.

The following examples are illustrative of the process of the inventionfor crosslinking polyolefin.

EXAMPLE 1

100 parts by weight of an extrusion molding grade of polyethylenepellets having a melt index of 0.2 and density of 0.922 g/cm³ are coatedby tumbling with 1.2 parts by weight of vinyltriethoxysilane havingdissolved therein 2.5 parts by weight of 1,1Di(tertbutylperoxy)-3,3,5-trimethylcyclohexane having a 0.1 hourhalf-life temperature of 128° C., 2.5 parts by weight of Di-tert-butylperoxide having a 0.1 hour half-life temperature of 164° C. and 3 partsby weight of dibutyltin dilaurate condensation catalyst, until all ofthe liquid is taken up. The composition is then extruded in a singlescrew extruder under the following conditions:

Temperature of screw: 60° C.

Temperature of barrel zone 1: 170° C.

Temperature of barrel zone 2: 220° C.

Screw speed: 20 r.p.m.

The residence time of the polyethylene in the machine is approximately 1to 2 minutes.

EXAMPLE 2

Composition:

Base polyethylene resin (Escorene LD 166 BA, with an MFL of 0.2 g/10 minat 190° C. under a load of 2.16 kg, and a density of 0.922 g/dm3): 98.8%by weight; and a premixed blend of silane, peroxide and catalyst (A-171vinyltrimethoxy silane: 1.11% by weight; 1,1Di(tertbutylperoxy)-3,3,5-trimethylcyclohexane: 0.028% by weight;Di-tert-butyl peroxide: 0.028% by weight; dibutyltin dilauratecondensation catalyst: 0.033% by weight): 1.2% by weight.

Process:

The pre-mixed blend of reactants was pre-soaked onto the polymer pelletsprior to feeding into the extruder by mixing at room temperature for 4hours. The thermoplastic polymer formulation was extruded on a Troestersingle screw extruder equipped with a barrier screw of a diameter of 45mm and a length of 25 l/d. No breaker plate was used and the screw speedwas set at 20 rpm. The feeding zone and screw were respectively cooledto 50° C. and 60° C. The barrel temperatures were set at 150° C. for thefirst zone with a regular increase until 220° C. in the last die zone.The resulting melt temperature was measured in the polymer at 226° C.

Example 2 developed a die pressure of 279 bar and gave a very smoothsurface finish to the extrudate. The tensile strength at break was 17.3MPa, the elongation at break was of 392%, and the hot-set measured at 15min at 200° C. under a load of 0.2 MPa was of 50% with a permanent setof 0%.

Comparative Example A

Composition:

Base polyethylene resin (Escorene LD 166 BA, with an MFL of 0.2 g/10 minat 190° C. under a load of 2.16 kg, and a density of 0.922 g/dm3): 98.8%by weight; and a standard but highly efficient commercially availablepremixed blend of silane, peroxide and catalyst containing only oneperoxide (Silcat RHE): 1.2% by weight.

Process:

The commercial blend of reactants was pre-soaked onto the polymerpellets prior to feeding into the extruder by mixing at room temperaturefor 4 hours. The thermoplastic polymer formulation was extruded,identically to example 2, on a Troester single screw extruder equippedwith a barrier screw of a diameter of 45 mm and a length of 25 l/d. Nobreaker plate was used and the screw speed was set at 20 rpm. Thefeeding zone and screw were respectively cooled to 50° C. and 60° C. Thebarrel temperatures were set at 150° C. for the first zone with aregular increase until 220° C. in the last die zone. The resulting melttemperature was measured in the polymer at 226° C.

Comparative Example A developed a die pressure of 295 bar and gave asmooth surface finish to the extrudate. The tensile strength at breakwas 17.6 MPa, the elongation at break was of 380%, and the hot-setmeasured at 15 min at 200° C. under a load of 0.2 MPa was of 50% with apermanent set of 0%.

This is a comparative working example of a new silane and multi-peroxideblend formulation as described in the application versus a standardhighly efficient commercial product containing a single peroxide.

The new blends are equally highly efficient, but develop less pressuredue to lower scortching and result in a better surface quality.

EXAMPLE 3

Composition:

Base polyethylene resin (Escorene LD 166 BA, with an MFL of 0.2 g/10 minat 190° C. under a load of 2.16 kg, and a density of 0.922 g/dm3): 86.9%by weight; a carbon black masterbatch (Black Plastback PE2737): 7% byweight; an anti-oxidant masterbatch (MBMM21085): 1.1% by weight; porousorganic carrier pellets (Pearlene 200HD): 3.5% by weight; and a premixedblend of silane, peroxide and catalyst (A-171 vinyltrimethoxy silane:91.5% by weight; 1,1 Di(tertbutylperoxy)-3,3,5-trimethylcyclohexane:1.5% by weight; tert-butyl cumyl peroxide: 2% by weight;di-tertbutylperoxide: 2% by weight; dibutyltin dilaurate condensationcatalyst: 3% by weight): 1.5% by weight.

Process:

The pre-mixed blend of reactants was absorbed into the porous organiccarrier pellets by mixing at room temperature during 10 minutes. Thepolymer base resin, carbon black masterbatch, anti-oxidant masterbatchand porous carrier pellets (including the pre-mixed blend of reactants)were fed into the extruder using a gravimetric blender. Thethermoplastic polymer formulation was extruded on a Troester singlescrew extruder equipped with a barrier screw of a diameter of 45 mm anda length of 25 l/d. No breaker plate was used and the screw speed wasset at 20 rpm. The feeding zone and screw were respectively cooled to50° C. and 60° C. The barrel temperatures were set at 150° C. for thefirst zone with a regular increase until 220° C. in the last die zone.

Example 3 gave a smooth surface finish to the extrudate. The hot-setmeasured at 15 min at 200° C. under a load of 0.2 MPa was of 50%.

Comparative Example B

Composition:

Base polyethylene resin (Escorene LD 166 BA, with an MFL of 0.2 g/10 minat 190° C. under a load of 2.16 kg, and a density of 0.922 g/dm3): 86.9%by weight; a carbon black masterbatch (Black Plastback PE2737): 7% byweight; an anti-oxidant masterbatch (MBMM21085): 1.1% by weight; porousorganic carrier pellets (Pearlene 200HD): 3.5% by weight; and a premixedblend of silane, peroxide and catalyst (A-171 vinyltrimethoxy silane:92.5% by weight; di-tertbutylperoxide: 4.5% by weight; dibutyltindilaurate condensation catalyst: 3% by weight): 1.5% by weight.Process:

The pre-mixed blend of reactants was absorbed into the porous organiccarrier pellets by mixing at room temperature during 10 minutes. Thepolymer base resin, carbon black masterbatch, anti-oxidant masterbatchand porous carrier pellets (including the pre-mixed blend of reactants)were fed into the extruder using a gravimetric blender. Thethermoplastic polymer formulation was extruded on a Troester singlescrew extruder equipped with a barrier screw of a diameter of 45 mm anda length of 25 l/d. No breaker plate was used and the screw speed wasset at 20 rpm. The feeding zone and screw were respectively cooled to50° C. and 60° C. The barrel temperatures were set at 150° C. for thefirst zone with a regular increase until 220° C. in the last die zone.

Comparative Example B gave a bad surface finish to the extrudate. Thesample broke during the hot-set test at 15 min at 200° C. under a loadof 0.2 MPa showing no thermomechanical resistance.

This is a comparative working example of a new silane and multi-peroxideblend formulation as described in the application versus a standardsingle-peroxide containing comparative formulation, used in a typicalcomplete thermoplastic polymer formulation intended for electricalwiring. These examples also use porous organic polyethylene carriers tofeed the blend of reactants.

Even at a higher peroxide content, a better surface quality is obtained.The higher peroxide content allows one to obtain the required hot-setperformance not achievable with the comparative single-peroxidecontaining product.

EXAMPLE 4

Composition:

Base polyethylene resin (Exxon LL 4004 EL, with an MFL of 0.33 g/10 minat 190° C. under a load of 2.16 kg, and a density of 0.922 g/dm3): 98.4%by weight; an anti-oxidant/color masterbatch: 0.6% by weight; and apremixed blend of silane, peroxide and catalyst (A-171 vinyltrimethoxysilane: 93.75% by weight; 1,1Di(tertbutylperoxy)-3,3,5-trimethylcyclohexane: 0.75% by weight;tert-butyl cumyl-peroxide: 1.5% by weight; di-tertbutylperoxide: 1.5% byweight; dibutyltin dilaurate condensation catalyst: 2.5% by weight):1.0% by weight.Process:

The polymer base resin and anti-oxidant/color masterbatch were fed intothe extruder using a gravimetric blender. The silane was injected inliquid form according to the Monosil process. The thermoplastic polymerformulation was extruded on a Nokia-Maillefer single screw extruderequipped with a barrier screw of a diameter of 120 mm and a length of 30l/d. No breaker plate was used and the screw speed was set at 18 rpm.The feeding zone and screw were cooled to 80° C. The barrel temperatureswere set at 150° C. for the first zone with a regular increase until215° C. in the last die zone.

Example 4 gave a smooth and very glossy surface finish to the extrudate.The tensile strength at break was 13.2 MPa, the elongation at break wasof 433%. The hot-set measured at 15 min at 200° C. under a load of 0.2MPa was of 70%.

Comparative Example C

Composition:

Base polyethylene resin (Exxon LL 4004 EL, with an MFL of 0.33 g/10 minat 190° C. under a load of 2.16 kg, and a density of 0.922 g/dm3): 98.2%by weight; an anti-oxidant/color masterbatch: 0.6% by weight; and apremixed blend of silane, peroxide and catalyst (A-171 vinyltrimethoxysilane: 93.75% by weight; 1,1Di(tertbutylperoxy)-3,3,5-trimethylcyclohexane: 0.75% by weight;tert-butyl cumyl peroxide: 1.5% by weight; di-tertbutylperoxide: 1.5% byweight; dibutyltin dilaurate condensation catalyst: 2.5% by weight):1.2% by weight.

Process:

The polymer base resin and anti-oxidant/color masterbatch were fed intothe extruder using a gravimetric blender. The silane was injected inliquid form according to the Monosil process. The thermoplastic polymerformulation was extruded on a Nokia-Maillefer single screw extruderequipped with a barrier screw of a diameter of 120 mm and a length of 30l/d. No breaker plate was used and the screw speed was set at 18 rpm.The feeding zone and screw were cooled to 80° C. The barrel temperatureswere set at 150° C. for the first zone with a regular increase until215° C. in the last die zone.

Comparative Example C gave a smooth and glossy surface finish to theextrudate. The tensile strength at break was 15.5 MPa, the elongation atbreak was of 495%. The hot-set measured at 15 min at 200° C. under aload of 0.2 MPa was of 45%.

This is a comparative working example of a new silane and multi-peroxideblend formulation as described in the application used at 2 differentloading levels in the compound. The conditions of processing weretypical of industrial productions using a Monosil process.

The efficiency of the silane and multi-peroxide blends remains high evenat very low use levels further reducing scorching and gel formationproblems. During the trials ran according to industrial processingconditions on a full production line, no defects (gels, pitts or otherirregularities) have been visible and a very smooth and glossy surfacewas obtained.

1. A process for producing a silane-crosslinked thermoplastic polyolefincomprising: a. providing a mixture of: (i) at least one silanepossessing an unsaturated organic function; (ii) at least two freeradical initiators, the first initiator having a first 0.1 hourhalf-life temperature and the second initiator having a second 0.1 hourhalf-life temperature being between 30° and 90° C. higher than saidfirst half-life temperature; (iii) at least one thermoplastic polyolefinwherein the thermoplastic polyolefin is at least one polyolefin selectedfrom the group consisting of high-pressure low-density polyethylene,medium/low-pressure high-density polyethylene, low-pressure low-densitypolyethylene, medium-density polyethylene, an ethylene-α-olefincopolymer, polypropylene, an ethylene-ethyl acrylate copolymer, anethylene-vinyl acetate copolymer, an ethylene-propylene copolymer, anethylene-propylene-diene terpolymer, an ethylene-butene copolymer,polymethyl-pentene-1, polybutene, chlorinated polyethylene, anethylene-vinyl acetate-chlorine terpolymer, and mixtures thereof; and,b. reacting the mixture of step (a) under reactive mechanical-workingconditions and exposure to moisture to provide crosslinked polyolefin.2. The process of claim 1 wherein the silane possesses the generalformula RR′SiY₂ wherein R represents a monovalently olefinicallyunsaturated hydrocarbon, each Y represents a hydrolysable organicradical and R′ represents an R radical or a Y radical.
 3. The process ofclaim 2 wherein the silane is vinyl triethoxysilane and/or vinyltrimethoxysilane.
 4. The process of claim 1 wherein the 0.1 hourhalf-life temperatures of the first free radical initiator is from about80° to about 160° C.
 5. The process of claim 1 wherein the 0.1 hourhalf-life temperature of the second free radical initiator is from about125° to about 190° C.
 6. The process of claim 1 wherein the first freeradical initiator is selected from the group consisting ofdi(2,4-dichlorobenzoyl) peroxide, tert-butyl peroxypivalate, dilauroylperoxide, dibenzoyl peroxide, tert-butyl peroxy-2-ethylhexanoate,1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane,di(tert-butylperoxy)cyclohexane, tert-butylperoxy-3,5,5-trimethylhexanoate, tert-butyl peroxyacetate, tert-butylperoxybenzoate, di-tert-amyl peroxide, dicumyl peroxide,di(tert-butylperoxyisopropyl)benzene and2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.
 7. The process of claim 1wherein the second free radical initiator is selected from the groupconsisting of tert-butyl peroxyacetate, tert-butyl peroxybenzoate,di-tert-amyl peroxide, dicumyl peroxide,di(tert-butylperoxyisopropyl)benzene,2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, tert-butyl cumyl peroxide,2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3 and di-tert-butylperoxide.
 8. The process of claim 1 wherein mixture (a) further includesat least one additional component selected from the group consisting ofcatalysts, stabilizers, fillers, antioxidants, processing aids, oils,plasticizers, pigments and lubricants.
 9. The process of claim 8 wherecatalyst is a metal carboxylate, an organic metal compound, an organicbase, or an acid.
 10. A process for producing a silane-crosslinkedthermoplastic polyolefin comprising: a. providing a mixture of: (i) atleast one silane possessing an unsaturated organic function, wherein thesilane is vinyl triethoxysilane and/or vinyl trimethoxysilane; (ii) atleast two free radical initiators, the first initiator having a first0.1 hour half-life temperature and the second initiator having a second0.1 hour half-life temperature being between 30° and 90° C. higher thansaid first half-life temperature; (iii) at least one thermoplasticpolyolefin; and, b. reacting the mixture of step (a) under reactivemechanical-working conditions and exposure to moisture to providecrosslinked polyolefin.
 11. The process of claim 10 wherein the 0.1 hourhalf-life temperatures of the first free radical initiator is from about80° to about 160° C.
 12. The process of claim 10 wherein the 0.1 hourhalf-life temperature of the second free radical initiator is from about125° to about 190° C.
 13. The process of claim 10 wherein the first freeradical initiator is selected from the group consisting ofdi(2,4-dichlorobenzoyl) peroxide, tert-butyl peroxypivalate, dilauroylperoxide, dibenzoyl peroxide, tert-butyl peroxy-2-ethylhexanoate,1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane,di(tert-butylperoxy)cyclohexane, tert-butylperoxy-3,5,5-trimethylhexanoate, tert-butyl peroxyacetate, tert-butylperoxybenzoate, di-tert-amyl peroxide, dicumyl peroxide,di(tert-butylperoxyisopropyl)benzene and2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.
 14. The process of claim 10wherein the second free radical initiator is selected from the groupconsisting of tert-butyl peroxyacetate, tert-butyl peroxybenzoate,di-tert-amyl peroxide, dicumyl peroxide,di(tert-butylperoxyisopropyl)benzene,2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, tert-butyl cumyl peroxide,2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3 and di-tert-butylperoxide.
 15. The process of claim 10 wherein mixture (a) furtherincludes at least one additional component selected from the groupconsisting of catalysts, stabilizers, fillers, antioxidants, processingaids, oils, plasticizers, pigments and lubricants.
 16. The process ofclaim 15 where catalyst is a metal carboxylate, an organic metalcompound, an organic base, or an acid.